Process for the preparation of a multicoat refinish

ABSTRACT

The present invention relates to a process for the preparation of a multicoat refinish, in which process a metallic basecoat is applied by spraying to the pretreated old finish and dried and then a clearcoat is applied to the basecoat and the clearcoat is dried together with the basecoat. The process is characterized in that the aqueous basecoat composition is applied such that 
     a) in a first spray pass the aqueous basecoat composition is applied to the pretreated area of the old finish until a boundary is reached, 
     b) the area of the old finish coated in step (a) is then coated in a second spray pass with the aqueous basecoat composition, 
     c) steps (a) and (b) of the process are repeated in the region of the old finish which adjoins the previously coated region of the old finish, again until a boundary is reached, and 
     d) process step (c) is carried out until the entire component to be coated is provided with a basecoat.

FIELD OF THE INVENTION

The present invention relates to a process for the preparation of amulticoat refinish, in which

1. the old finish is prepared by cleaning, sanding and, if desired,applying a surfacer and/or filler material,

2. an aqueous basecoat composition is applied, by means of sprayapplication, which contains metallic pigments and/or effect pigments,

3. a polymer film is formed from the composition applied in step (2),

4. a suitable transparent topcoat composition is applied to theresulting basecoat, and

5. subsequently the topcoat together with the basecoat is dried attemperatures of up to 140° C., preferably at temperatures below 100° C.and particularly preferably at temperatures of from 60° to 80° C.

BACKGROUND OF THE INVENTION

The substrates suitable for refinish are either an old finish(production-line finish or repair finish), new parts which have beenprimed (mostly by electrodeposition, especially in the area of thefinishing of heavy goods vehicle bodies), and also steel, aluminum, zincplate or plastic. The choice of suitable coating materials and theprocess employed for the preparation of a refinish are determined by thesurface to be finished and by the requirements of the customer.

A multicoat refinish is conventionally prepared by thorough cleaning ofthe damage site, sanding, treatment, if desired, with a surfacer andapplication of a repair filler to the area to be coated. In the case ofall-over finishes, the entire old finish to be coated is pretreatedanalogously. Subsequently the area pretreated in this way is finished.

In this context, however, the preparation of metallic refinishes is aparticular problem, because the color and brightness of the effectdepend heavily on the manner of processing. Decisive factors at playhere include the nozzle width of the spray gun and the spray pressure.The nature of dilution and the spray viscosity also influence color andeffect. Especially when using aqueous basecoat compositions, cloudingand similar effects are often observed.

The object of the present invention was therefore to provide a processfor the preparation of a refinish having a very good quality, even inthe case of critical colors. This means that changes in color, cloudingand the like are to be avoided.

SUMMARY OF THE INVENTION

This object is surprisingly achieved by a process of the type mentionedin the FIELD OF THE INVENTION section, which is characterized in that instep (2) of the process the aqueous basecoat composition is applied suchthat

a) in a first spray pass the aqueous basecoat composition is applied tothe pretreated area of the old finish until a boundary is reached,

b) the area of the old finish coated in step (a) is then coated in asecond spray pass with the aqueous basecoat composition,

c) steps (a) and (b) of the process are repeated in the region of theold finish which adjoins the previously coated region of the old finish,again until a boundary is reached, and

d) process step (c) is carried out until the entire component to becoated is provided with a basecoat.

Surprisingly, using the process according to the invention it ispossible to obtain refinishes in which the problems that usually occurare avoided or at least sharply reduced. In particular, the cloudingwhich is frequently observed in the conventional refinish processes isavoided. In this context, it is significant that these outstandingresults can also be achieved while using aqueous metallic basecoats. Afurther advantage is the saving in time brought about by the fact thatthere is only a brief flash-off time and, particularly preferably, noflash-off time at all between the first and second spray pass.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

This process according to the invention for the preparation of amulticoat refinish can be applied to a wide variety of substrates. Inthis context, it is irrelevant whether the systems to be provided withthe refinish are conventional or are water-dilutable.

In order to carry out the process according to the invention, the oldfinish is first prepared to receive the refinish. Conventionally, forthis purpose, the old finish is first thoroughly cleaned, for examplewith a silicone stripper, and roughened with abrasive paste orsandpaper. Thereafter, the area pretreated in this way is conventionallycleaned again and degreased. In addition to this, however, it is alsopossible to precoat the old finish, for example, using a solution ofadhesion promoter. This preparation of the old finish, however, is knownand therefore requires no further description.

If desired, the appropriately pretreated old finish can then haveapplied to it an aqueous or water-dilutable or else a conventionalcoating composition. For this operation, it is conventional to applyhighly diluted clearcoats that may be aqueous, water dilutable, andconventional. If conventional clearcoats are applied, then drying mustbe carried out before the basecoat is applied. This is conventionallyeffected by heating the clearcoat film to a temperature of below 140°C., preferably below 80° C., for a period of 5-60 min. If aqueouscoating compositions are employed for this step of the process, then itis possible, if desired to do without a baking step: after a briefflash-off time of 5-50 min. the basecoat composition can be applieddirectly. If the old finish is presprayed with such a clearcoat, then itis possible if desired, to do without the sanding of the old finish.Conventionally, however, the above-described pretreatment of the oldfinish is carried out by cleaning and sanding. Aqueous clearcoats thatare suitable for this pretreatment of the old finish are described, forexample, in DE-A-40 09 000. In addition, however, it is also possible toemploy the conventional clearcoats that are normally employed for thispurpose, as marketed, for example, by the company Glasurit GmbH.

It is essential to the invention that a basecoat composition containingmetallic and/or effect pigments is then applied, using a specificapplication technique, to the appropriately prepared old finish.

In the process according to the invention the aqueous basecoatcomposition is initially applied, in a first spray pass, to thepretreated old finish until a boundary is reached. Directly subsequentto this (i.e. without intermediate drying) the aqueous basecoatcomposition is applied, in a second spray pass, to the previously coatedparts of the old finish, again until the boundary is reached. This meansthat, for example, initially the complete roof of a car is coated, in afirst spray pass, with the aqueous basecoat composition. Directlysubsequent to this the roof is then coated once more, in a second spraypass, with the aqueous basecoat composition.

In the next step of the process according to the invention the region ofthe old finish which adjoins the previously coated region of the oldfinish is coated with the aqueous basecoat composition again until aboundary is reached, in accordance with the previous directions, in twospray passes that follow in direct succession. This means, for example,that following the above-described coating of the roof, one wing of thecar is first coated with the aqueous basecoat composition in two spraypasses which follow in direct succession. The process step (c) describedabove is carried out until the entire component to be coated is providedwith a basecoat. In the example described above, this means, forexample, that after the coating of the wing, the adjacent door is thenprovided in turn with the basecoat in 2 spray passes.

This process step of area-by-area coating with the basecoat compositionis repeated until the entire component to be coated is coated with thebasecoat.

Between the first spray pass of the basecoat composition and the secondspray pass of the aqueous basecoat composition that follows it, there isusually a flash-off time of less than 30 min, particularly preferably aflash-off time of less than 20 min. It is very particularly preferred tooperate without a flash-off time between the first and the second spraypasses.

The application of the basecoat composition is carried out under theconventional conditions. This means that, for example, a normal or onlya slightly reduced spray-gun pressure is employed. The spray gun inputpressure is preferably between 3.0 and 4.5 bar. The output pressure ofthe spray gun varies in this case depending on the spray gun used. Whenan HVLP (high-volume low-pressure) gun is used, it is, for example,between 0.6 and 0.7 bar. For the application of the basecoatcomposition, it is appropriate to use all spray guns that areconventionally employed for the application of basecoats, for examplethe abovementioned HVLP guns, but also normal pneumatic,high-performance spray guns (e.g., SATA-Jet 1,4E).

Aqueous basecoat compositions which are suitable for this process areall those that contain metallic and/or effect pigments and areconventionally employed for refinishing. Particularly suitable basecoatcompositions are those containing as binder at least one polyurethaneresin. Thus, for example, the aqueous basecoat compositions described inDE-A-40 09 000 are suitable. Also suitable are the aqueous basecoatcompositions prepared using a mixer system.

Examples of suitable aqueous basecoat compositions are those that areprepared using the mixer system described in DE-A-41 10 520, in thenonprior publication German Patent Application P 42 32 721.0 and in thenonprior publication German Patent Application P 42 32 717.2.

After formation of a polymer film from the basecoat composition,preferably after drying of the basecoat at temperatures of below 140°C., preferably at temperatures below 80° C., for a period of 5-60 min,if desired after a brief cooling-down time of in general at least 5 min,a suitable transparent topcoat composition is applied to the basecoat.The dry film thickness of the topcoat is in general between 30 and 100μm. The dry film thickness of the basecoat is in general between 10 and25 μm.

Both 1- or 2-component clearcoats present as organic solutions and thosein aqueous form are suitable as topcoat compositions. Clearcoatsfrequently employed are those based on an acrylate copolymer thatcontains hydroxyl groups and on a polyisocyanate. Examples of suchclearcoats are described in Patent Applications DE-A-34 12 534, DE-A-3609 519, DE-A-37 31 652, and DE-A-38 23 005. Also suitable are themoisture-curing clearcoats based on polyaddition polymers, containingalkoxysilane or aryloxysilane units, that are described in theInternational Patent Application having the international publicationno. WO 88/02010.

After a flash-off time of about 5 minutes, if necessary, the topcoat isthen dried, together, if appropriate, with the basecoat, at temperaturesof up to 140° C., preferably at temperatures below 100° C., andparticularly preferably at temperatures of from 60° to 80° C., for aperiod of 5-120 min.

Substrates that are suitable for the preparation of the multicoatrefinish according to the invention are a very wide variety ofsubstrates, for example old finishes (production-line finish or repairfinish) and new parts that have been primed (mostly byelectrodeposition, especially in the area of the finishing of heavygoods vehicle bodies). The process is employed in particular for therefinishing of entire components.

The invention is now illustrated in more detail in the followingexamples. In these examples, all parts and percentages are by weightunless expressly stated otherwise. The substrate used is a multicoat,simulated old finish, as is conventional in the production-linefinishing of cars. In this context, it is irrelevant whether the paintsystems involved are based on conventional or water-dilutable systems.

Simulation of an old finish

The substrate used is an electrodeposition-primed steel panel providedwith a conventional commercial filler material based on amelamine-crosslinked polyester resin (FC60-7133 from BASF Lacke+FarbenAG, Munster; dry film thickness 40 μm), a conventional commercialmetallic basecoat based on cellulose acetobutyrate (AE54-9153 from BASFLacke+Farben AG, Munster; dry film thickness 15 μm), and a conventionalcommercial clearcoat based on isocyanate-crosslinked acrylates thatcontain hydroxyl groups (AF23-0185 from BASF Lacke+Farben AG, Munster;dry film thickness 60 μm). After conventional drying (60° C., 30 min),the coated panel is additionally stored for several hours at elevatedtemperature, for example 60° C., in order to age the finish. The oldfinish simulated in this way is first cleaned, then roughened withsandpaper and cleaned again. The commercial 1-component primer-surfacerGlassohyd® 76-86W from BASF Lacke+Farben AG, Munster, is then applied in2 spray passes to the old finish pretreated in this way, and the finishis dried at 60° C. for 30 min. This is followed by the application ofthe aqueous commercial refinish basecoat VW mint-met. VWL-B6/00U fromBASF Lacke+Farben AG, Munster (dry film thickness 15 μm). Application iseffected using the HVLP (high-volume low-pressure) spray gun SATA Jet BNR92 (nozzle width 1.3 mm) at a spray gun input pressure of 4.5 bar(spray gun output pressure 0.7 bar). In order to coat the car bodyprovided with the above-described old finish, the roof is first sprayedto completion in 2 spray passes. Then spraying is begun on the wing andthis spraying is completed with 2 spray passes. The next area (forexample the door) is then sprayed to completion with 2 spray passes.

This process is continued area by area until the entire body has beencompletely coated with the basecoat. The basecoat is then dried for 10min at 60° C. and allowed to cool for 5 min. The commercial 2-componentclearcoat based on isocyanate-crosslinked acrylates that containhydroxyl groups (AF 23-0185 plus SC 29-0173 plus SV 41-0391 from BASFLacke+Farben AG, Munster; mixing ratio 2:1:0.6) is then applied to thebasecoat. This clearcoat is applied at a dry film thickness of 60 μm.After a short flash-off time of 5 min the topcoat is subsequently driedat 60° C. for 30 min.

The coating obtained in this way exhibits outstanding surfaceproperties. In particular, the resulting coating exhibits advantages inrespect of clouding compared with coatings in which the complete carbody is coated with the metallic basecoat in one spray pass and, afterthe entire body has been completed, the basecoat is again applied to theentire body in a second spray pass.

We claim:
 1. A process for the preparation of a multicoat refinish of acomponent, comprising the steps of:a. preparing the old finish locatedon the component by cleaning, sanding, and, if desired, applying atleast one member of the group consisting of surfacers, filler materials,and mixtures thereof; b. applying by spray application an aqueousbasecoat composition containing at least one member of the groupconsisting of metallic pigments, effect pigments, and mixtures thereof;c. forming a basecoat film from said applied basecoat composition; d.applying a suitable transparent topcoat composition to said basecoat;and e. drying said topcoat together with said basecoat at temperaturesof up to 140° C.;wherein said aqueous basecoat composition applicationcomprises the steps of: (i) applying the aqueous basecoat composition intwo spray passes to a first area of the pretreated old finish of thecomponent until a boundary of said first area is reached; (ii) repeatingstep (i) in a second area of the pretreated old finish adjoining saidfirst area of the old finish coated in step (i) untii a new boundary isreached; and (iii) repeating step (ii) until the component to be coatedis provided with a basecoat.
 2. A process according to claim 1, whereinthere is a flash-off time of less than 30 minutes between the two passesin step (i).
 3. A process according to claim 1, wherein the basecoatfilm is formed by the basecoat composition being dried at temperaturesof below 140° C. for a period of from 5 to 60 minutes.
 4. A processaccording to claim 1, wherein the aqueous basecoat composition containsas binder at least one polyurethane resin.
 5. A process according toclaim 1, wherein the component is at least a part of a vehicle body. 6.A process according to claim 1, wherein the topcoat and the basecoat aredried together at temperatures below 100° C.
 7. A process according toclaim 1, wherein the topcoat and the basecoat are dried together attemperatures of from 60° to 80° C.
 8. A process according to claim 2,wherein there is no flash-off time between the two passes in step (i).9. A process according to claim 3, wherein the basecoat composition isdried at temperatures of below 80° C.